A reduction in emission of carbon dioxide has been strongly desired for environmental protection in recent years. The automobile industry has placed great expectation on electric vehicles (EV) and hybrid electric vehicles (HEV) introduced to reduce emission of carbon dioxide, and thus has been extensively developing secondary batteries for driving motors, which are the key to practical use of EVs and HEVs. Among those secondary batteries, lithium ion secondary batteries have received attention because high energy density and high output power density can be attained.
A typical lithium ion secondary battery includes electrodes composed of a positive electrode current collector onto which a positive electrode active material is applied together with a binder and a negative electrode current collector onto which a negative electrode active material is applied together with a binder. A bipolar battery includes a bipolar electrode composed of a current collector having a positive electrode layer formed by applying a positive electrode active material and the like together with a binder onto one surface of the current collector and a negative electrode layer formed by applying a negative electrode active material and the like together with a binder onto the other surface thereof.
Such lithium ion secondary batteries typically include metallic foil (metallic current collecting foil) as current collectors. The so-called resin current collectors composed of resins containing metal powder have been proposed instead of the metallic foil (see Patent Literature 1, for example) in recent years. Such resin current collectors are lighter than the metallic current collecting foil and expected to improve output power per unit weight of a battery.
Unfortunately, the traditional resin current collectors contain conductive fillers insufficiently dispersed, reducing the performance of the battery, such as charge and discharge characteristics.
Dispersion of the conductive fillers is slightly but insufficiently enhanced by commonly known dispersants (such as carboxymethyl cellulose and surfactants). Accordingly, a resin current collector containing a conductive filler uniformly dispersed is desired.